The present invention is related generally to force transducers utilized in vehicle wheel balancing systems to measure imbalance forces in a rotating body, and in particular, to a force transducer for a vehicle wheel balancing system incorporating a automatically calibrating piezoelectric force transducer.
The determination of imbalance in vehicle wheel assemblies is generally carried out by an analysis of the vehicle wheel assembly with reference to phase and amplitude of any mechanical vibrations caused by a rotation of the unbalanced masses present in the vehicle wheel assembly. These mechanical vibrations are measured as motions, forces, or pressures by means of transducers, which convert the mechanical effects to electrical signals. Each signal is the combination of fundamental oscillations caused by imbalance and noise.
Generally, vehicle wheel balancing systems include a shaft adapted for receiving a vehicle wheel assembly, capable of being rotationally driven about a longitudinal axis so as to rotate a vehicle wheel assembly which is removably mounted thereon. A rotation sensor assembly is provided for measuring rotation of the shaft about the longitudinal axis, and various force transducers are operatively connected to the shaft for measuring the mechanical vibrations resulting from the rotating imbalances in the vehicle wheel assembly. A control circuit regulates the driven rotation, and determines, from signals representative of the mechanical vibrations measured by the force transducers, any required imbalance correction weight amounts and placement locations on the vehicle wheel assembly necessary to correct any measured vibrations resulting from the imbalances present in the vehicle wheel assembly.
Typically a vehicle wheel balancer contains two force transducers which sense the mechanical vibrations as imbalance forces during a rotation of a vehicle wheel assembly on the shaft. These force transducers are usually based on ceramic piezoelectric devices that generate an electrical voltage output in an amount proportional to the force applied to the devices. The electrical voltage is then read by the control circuit of the balancer electronics, enabling computation of an imbalance value for the vehicle wheel assembly.
The output of piezoelectric devices change with time, temperature, preload, and load history. Compensating for these changes requires the user to perform periodic manual calibration procedures to ensure continued accuracy. Most vehicle wheel balancers require this periodic calibration procedure to be performed. Failure on the part of an operator to perform the required periodic calibrations, or improper calibrations, can result in errors in determining the imbalance condition of a vehicle wheel assembly, leading to the incorrect application of imbalance correction weights, repeated attempts to balance (“chase spins”) the vehicle wheel, and at worst, unsatisfied customers (“comebacks”).
Accordingly, it would be advantageous to provide a vehicle wheel balancing system with a force transducer assembly which does not require the operator to periodically perform a manual calibration procedure, thereby eliminating the need to remind an operator for periodic calibrations, and reducing the opportunity for the introduction of operator errors.